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MySQL 5.7 Reference Manual  /  ...  /  Fine-Tuning MySQL Full-Text Search

12.9.6 Fine-Tuning MySQL Full-Text Search

MySQL's full-text search capability has few user-tunable parameters. You can exert more control over full-text searching behavior if you have a MySQL source distribution because some changes require source code modifications. See Section 2.9, “Installing MySQL from Source”.

Full-text search is carefully tuned for effectiveness. Modifying the default behavior in most cases can actually decrease effectiveness. Do not alter the MySQL sources unless you know what you are doing.

Most full-text variables described in this section must be set at server startup time. A server restart is required to change them; they cannot be modified while the server is running.

Some variable changes require that you rebuild the FULLTEXT indexes in your tables. Instructions for doing so are given later in this section.

Configuring Minimum and Maximum Word Length

The minimum and maximum lengths of words to be indexed are defined by the innodb_ft_min_token_size and innodb_ft_max_token_size for InnoDB search indexes, and ft_min_word_len and ft_max_word_len for MyISAM ones.


Minimum and maximum word length full-text parameters do not apply to FULLTEXT indexes created using the ngram parser. ngram token size is defined by the ngram_token_size option.

After changing any of these options, rebuild your FULLTEXT indexes for the change to take effect. For example, to make two-character words searchable, you could put the following lines in an option file:


Then restart the server and rebuild your FULLTEXT indexes. For MyISAM tables, note the remarks regarding myisamchk in the instructions that follow for rebuilding MyISAM full-text indexes.

Configuring the Natural Language Search Threshold

For MyISAM search indexes, the 50% threshold for natural language searches is determined by the particular weighting scheme chosen. To disable it, look for the following line in storage/myisam/ftdefs.h:


Change that line to this:


Then recompile MySQL. There is no need to rebuild the indexes in this case.


By making this change, you severely decrease MySQL's ability to provide adequate relevance values for the MATCH() function. If you really need to search for such common words, it would be better to search using IN BOOLEAN MODE instead, which does not observe the 50% threshold.

Modifying Boolean Full-Text Search Operators

To change the operators used for boolean full-text searches on MyISAM tables, set the ft_boolean_syntax system variable. (InnoDB does not have an equivalent setting.) This variable can be changed while the server is running, but you must have the SUPER privilege to do so. No rebuilding of indexes is necessary in this case. See Section 5.1.5, “Server System Variables”, which describes the rules governing how to set this variable.

Character Set Modifications

For the built-in full-text parser, you can change the set of characters that are considered word characters in several ways, as described in the following list. After making the modification, rebuild the indexes for each table that contains any FULLTEXT indexes. Suppose that you want to treat the hyphen character ('-') as a word character. Use one of these methods:

  • Modify the MySQL source: In storage/innobase/handler/ (for InnoDB), or in storage/myisam/ftdefs.h (for MyISAM), see the true_word_char() and misc_word_char() macros. Add '-' to one of those macros and recompile MySQL.

  • Modify a character set file: This requires no recompilation. The true_word_char() macro uses a character type table to distinguish letters and numbers from other characters. . You can edit the contents of the <ctype><map> array in one of the character set XML files to specify that '-' is a letter. Then use the given character set for your FULLTEXT indexes. For information about the <ctype><map> array format, see Section 10.12.1, “Character Definition Arrays”.

  • Add a new collation for the character set used by the indexed columns, and alter the columns to use that collation. For general information about adding collations, see Section 10.13, “Adding a Collation to a Character Set”. For an example specific to full-text indexing, see Section 12.9.7, “Adding a Collation for Full-Text Indexing”.

Rebuilding InnoDB Full-Text Indexes

If you modify full-text variables that affect indexing (innodb_ft_min_token_size, innodb_ft_max_token_size, innodb_ft_server_stopword_table, innodb_ft_user_stopword_table, innodb_ft_enable_stopword, ngram_token_size you must rebuild your FULLTEXT indexes after making the changes. Modifying the innodb_ft_min_token_size, innodb_ft_max_token_size, or ngram_token_size variables, which cannot be set dynamically, require restarting the server and rebuilding the indexes.

To rebuild the FULLTEXT indexes for an InnoDB table, use ALTER TABLE with the DROP INDEX and ADD INDEX options to drop and re-create each index.

Optimizing InnoDB Full-Text Indexes

Running OPTIMIZE TABLE on a table with a full-text index rebuilds the full-text index, removing deleted Document IDs and consolidating multiple entries for the same word, where possible.

To optimize a full-text index, enable innodb_optimize_fulltext_only and run OPTIMIZE TABLE.

mysql> set GLOBAL innodb_optimize_fulltext_only=ON;
Query OK, 0 rows affected (0.01 sec)

mysql> OPTIMIZE TABLE opening_lines;
| Table              | Op       | Msg_type | Msg_text |
| test.opening_lines | optimize | status   | OK       |
1 row in set (0.01 sec)

To avoid lengthy rebuild times for full-text indexes on large tables, you can use the innodb_ft_num_word_optimize option to perform the optimization in stages. The innodb_ft_num_word_optimize option defines the number of words that are optimized each time OPTIMIZE TABLE is run. The default setting is 2000, which means that 2000 words are optimized each time OPTIMIZE TABLE is run. Subsequent OPTIMIZE TABLE operations continue from where the preceding OPTIMIZE TABLE operation ended.

Rebuilding MyISAM Full-Text Indexes

If you modify full-text variables that affect indexing (ft_min_word_len, ft_max_word_len, or ft_stopword_file), or if you change the stopword file itself, you must rebuild your FULLTEXT indexes after making the changes and restarting the server.

To rebuild the FULLTEXT indexes for a MyISAM table, it is sufficient to do a QUICK repair operation:

mysql> REPAIR TABLE tbl_name QUICK;

Alternatively, use ALTER TABLE as just described. In some cases, this may be faster than a repair operation.

Each table that contains any FULLTEXT index must be repaired as just shown. Otherwise, queries for the table may yield incorrect results, and modifications to the table will cause the server to see the table as corrupt and in need of repair.

If you use myisamchk to perform an operation that modifies MyISAM table indexes (such as repair or analyze), the FULLTEXT indexes are rebuilt using the default full-text parameter values for minimum word length, maximum word length, and stopword file unless you specify otherwise. This can result in queries failing.

The problem occurs because these parameters are known only by the server. They are not stored in MyISAM index files. To avoid the problem if you have modified the minimum or maximum word length or stopword file values used by the server, specify the same ft_min_word_len, ft_max_word_len, and ft_stopword_file values for myisamchk that you use for mysqld. For example, if you have set the minimum word length to 3, you can repair a table with myisamchk like this:

myisamchk --recover --ft_min_word_len=3 tbl_name.MYI

To ensure that myisamchk and the server use the same values for full-text parameters, place each one in both the [mysqld] and [myisamchk] sections of an option file:



An alternative to using myisamchk for MyISAM table index modification is to use the REPAIR TABLE, ANALYZE TABLE, OPTIMIZE TABLE, or ALTER TABLE statements. These statements are performed by the server, which knows the proper full-text parameter values to use.

User Comments
  Posted by John Navratil on May 10, 2007
How I added '-' to the list of word characters:

The documentation is weak in two regards: (1) it doesn't explain how to modify the map and (2) it doesn't touch on the implications of doing so. I'll try to solve (1), but cannot begin to speak to (2)

The charsets files exist at the location specified by the "character_sets_dir" system variable (use SHOW VARIABLES to see this) and is typically compiled in as "/usr/share/mysql/charsets". The name of the file is given by the "character_set_...' variables. Typically the default is "latin1". Thus the file I needed to change was /usr/share/mysql/charsets/latin1.xml

The <ctype><map> is the one we are after (other maps are "upper", "lower", "unicode" and the various collation maps).

The "ctype" map differs from the others in that is has a leading 0x00 before the character map, the meaning of which is unclear to me. Each entry of the map appears to classify the corresponding character according to the following bitmask:

0x01 Upper-case word character
0x02 Lower-case word character
0x04 Decimal digit
0x08 Printer control (Space/TAB/VT/FF/CR)
0x10 Not-white, not a word
0x20 Control-char (0x00 - 0x1F)
0x40 Space
0x80 Hex digit (0-9, a-f, A-F)

In my case, I needed the dash '-', but nothing else, so I altered the corresponding character position (0x2D - third row, third from the right) from 0x10 (Not-white, not a word) to 0x01 (Upper-case word).

There is little on the web to address this, but some commentary in the forums suggested that this was NOT the way to do this, but rather to write ones own full-text engine as the changing of the <ctype> map has implications for the SQL parser. This may be true, but I suspect SQL parsing would require a stricter classification of characters. The SQL statement "SELECT a-b FROM test" worked for me after this change.

Altering latin1.xml and restarting the server had the desired result.

Finally, there does not appear to be a way to create a new character set or collation without recompiling. If this is true, it might be desirable for the standard distribution to include a "custom" character set for just this sort of thing.
  Posted by Sebastien Salou on July 4, 2007
Based on your example with the dash `-`, I had a look to make the single quote `'` (which is a word character by default), a non word character.
I had a look on a ascii table, the single quote is corresponding to the hexadecimal value 27.
I opened the file share/mysql/charsets/latin1.xml, I went to the upper map (0x27 is actually on the 3rd rows, 8th col from the left).
I went to this position in the ctype map, and surprised !!! This character is already set to 0x10 Not-white, not a word whereas it is a word character during tests !

From there, I'm pretty lost. Why the single quote is not detected as a non word as it should be ?

Modifying the mysql source in myisam/ftdefs.h works.
I modified the line #define misc_word_char(X) ((X)=='\'')
Is it the only way ?

Sebastien Salou
  Posted by Scott Noyes on November 19, 2009
John Navratil very nearly has it, but to get fulltext to treat characters as words, you also have to add a new collation. I've written it up at
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